The surface acoustic wave (SAW) sensor offers advantages in that it is wireless, passive, small and has varying embodiments for different sensor applications. Surface acoustic wave sensors are capable of measuring physical, chemical and biological variables and have the ability to operate in harsh environments. In addition, there are a variety of ways of encoding the sensed data information for retrieval. Single sensor systems can typically use a single carrier RF frequency and a simple device embodiment, since tagging is not required. In a multi-sensor environment, it is necessary to both identify the sensor as well as obtain the sensed information. The SAW sensor then becomes both a sensor and a tag and must transmit identification and sensor information simultaneously.
Known SAW devices include delay lines and resonator-based oscillators, differential delay lines, and devices utilizing multiple reflective. Single sensor systems can typically use a single carrier frequency and a simple coding technique, since tagging is not required. However, there are advantages of using spread spectrum techniques for device interrogation and coding, such as enhanced processing gain and greater interrogation power.
The use of orthogonal frequencies for a wealth of communication and signal processing applications is well known to those skilled in the art. Orthogonal frequencies are often used in an M-ary frequency shift keying (FSK) system. There is a required relationship between the local, or basis set, frequencies and their bandwidths which meets the orthogonality condition. If adjacent time chips have contiguous local stepped frequencies, then a stepped chirp response is obtained.
Other known SAW devices include delay line and resonator-based oscillators, differential delay lines, and devices utilizing multiple reflective structures where the reflector length determines a single chip length. The amplitude, phase and delay of each chip can be different from adjacent chips and the sum of all chips yield the code sequence. In this serial approach, the greater the number of codes required, the longer the length of the device.
Known prior art includes U.S. Patent Application No. 2005/0100338 which teaches a two-dimensional wavelength/time optical CDMA system employing balanced-modified pseudo random noise matrix codes. Through an inverse-exclusive OR operation of a pair of modified PN code, the balanced codes are generated as optical CDMA codes in the form of a new matrix. When the codes are applied to an optical CDMA system to perform encoding and decoding, if the same number of channels as the number (M−1) of subgroups of the codes are connected, the system becomes an MAI-free system, and even if the number of channels connected is twice the number of the subgroups, an error-free system can be established. Accordingly, the number of channels that can be used simultaneously is doubled compared to the prior art method such that the economical efficiency of the optical CDMA system improves.
U.S. Patent Application No. 2008/0156100 published on Jul. 3, 2008 teaches an acoustic wave sensor array device for the detection, identification, and quantification of chemicals and biological elements dispersed in fluids. The sensor array device is capable of the simultaneous characterization of a fluid for multiple analytes of interest. A substrate has a plurality of channels formed therein and a sensor material layer applied in a bottom of the channels. The sensor material layer has a shear acoustic wave speed lower than a shear acoustic wave speed in said substrate. The channels may have the same material in each channel or different materials in at least two of the channels. A surface acoustic wave transducer and at least one surface acoustic wave reflector, or at least two transducers is formed on a surface of the substrate opposite the channels at a portion of the substrate that is thinned by the channels, so that the acoustic tracks of the surface acoustic wave device extend along the channels. The response of the surface acoustic wave depends on the response of the sensor material to a sensed fluid supplied to the channels.
U.S. Pat. No. 7,817,707 issued Oct. 19, 2010 teaches an apparatus for generating a ranging pseudo noise (PN) code used in a base station of a portable internet system of an orthogonal frequency division multiplexing access scheme, wherein a ranging pseudo noise mask value is generated using a cell ID number, and then the generated ranging pseudo noise mask value is stored in a memory. A final ranging PN code is generated using the stored ranging PN mask value and a status of a pseudo random binary sequence for generating a ranging PN code. With such a structure, the maximal 256-numbered ranging PN code values can be obtained simultaneously with each 144 bit-length.
Patents and patent applications by an inventor of the present invention, and assigned to the same assignee, and which are incorporated by reference, include U.S. Pat. Nos. 7,642,898, 7,777,625, 7,825,805, and 7,623,037. U.S. Pat. No. 7,642,898 issued on Jan. 5, 2010 to Malocha which teaches orthogonal frequency coding for surface acoustic wave identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, a pseudo noise sequence is applied to the OFC for increased security. An OFC technique is applied to the SAW tag using periodic reflector gratings for responding to an orthogonal interrogation signal to transmit the sensor identification and sensed data. A transceiver interrogates the sensor with a stepped chirp corresponding to the orthogonal frequency coded chip frequency response, receives a response from the SAW device, applies an oppositely stepped chirp to the response and then uses matched filtering to produce a compressed pulse. The orthogonal frequency coding technique has an inherent advantage of processing gain, code division multiple access, spread spectrum and security.
U.S. Pat. No. 7,777,625 issued on Aug. 17, 2010 to Puccio and Malocha, which discloses a weighted surface acoustic wave reflector gratings for coding identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, the weighted reflectors are variable while in another embodiment the reflector gratings are apodized. The weighting technique allows the designer to decrease reflectively and allows for more chips to be implemented in a device and, consequently, more coding diversity. As a result, more tags and sensors can be implemented using a given bandwidth when compared with uniform reflectors. Use of weighted reflector gratings with OFC makes various phase shifting schemes possible, such as in-phase and quadrature implementations of coded waveforms resulting in reduced device size and increased coding.
U.S. Pat. No. 7,825,805 issued on Nov. 2, 2010 to Malocha, which teaches systems, devices and methods for providing an orthogonal frequency coding technique for surface acoustic wave sensors incorporating the use of multiple parallel acoustic tracks to provide increased coding by phase shifting and delaying a code sequence. The surface acoustic wave sensor includes parallel tracks with multiple reflectors with differing delay offsets to form a complex code sequence. The reflectors may be uniform, but alternatively could include fingers withdrawn, have reflector position modulation, differing frequencies or be spatially weighted.
U.S. Pat. No. 7,623,037 issued on Nov. 24, 2011 to Malocha discloses a SAW sensor or tag having multiple transducer/antenna pairs each having a different center frequency. The bandwidth of each transducer/antenna pair is inversely proportional to the number of transducer/antennas pairs used and the bandwidth is the sum of the bandwidth of the transducer/antenna pairs. Implementing a SAW sensor or tag with multiple transducer/antenna pairs significantly reduces device losses and improves the performance of the device since the individual transducer/antenna pair's fractional bandwidth is reduced by the ratio of the system bandwidth to the number of transducer antenna pairs used in the sensor.
To solve the problems associated with the prior art systems, methods and systems of the present invention provides a novel type surface acoustic wave devices with on-off capabilities for passive wireless surface acoustic wave devices.